We split disks into two groups.
You get a limited amount of backed up storage, and, this is limited by us being able to actually backup that data.
You get a much much less limited amount of unbackedup storage.
This works in an environment where we don't have regulatory requirements to save and audit everything, and, we have users who can make the distinction between important and not so important storage.
If your environment isn't so relaxed you might want to look into Sun's SAM/QFS which lets you push snapshots offsite. I'm sure that other vendors have similar products. I have friends in heavily regulated environments who use this and are happy.